Interpretive Summary: Advances in phylogeography are of great value for understanding the population structure and origins of invasive genotypes. Such insights provide constructive information for current or future biological control research efforts. In this study, we investigate a highly variable chloroplast DNA (cpDNA) marker for populations of the weed Lepidium draba (Brassicaceae) in its native Eurasian and invasive USA ranges. We sequenced DNA from 684 individuals from Eurasia and the USA and found 41 different haplotypes. Our comparison between the native and invasive ranges found little reduction in measures of allelic richness (A) and haplotype diversity (h) since introduction into the USA, as well as little evidence of regional geographic structuring of haplotypes in neither Eurasia nor the USA. Assignment tests indicated most likely origins of many invasive haplotypes, though some origins were found outside of the putative native range of the species. Exact locations are identified for a diverse set of invasive haplotypes which can be used in ongoing host-specificity tests of potential biological control agents. Biological control efforts should be prepared to contend with a genetically diverse invasion derived most likely from multiple Eurasian origins.

Technical Abstract:
Advances in phylogeography are of great value for understanding the population structure and origins of invasive genotypes. Such insights provide constructive information for current or future biological control research efforts. In this study, we investigate a highly variable chloroplast DNA (cpDNA) marker for populations of the weed Lepidium draba (Brassicaceae) in its native Eurasian and invasive USA ranges. We sequenced DNA from 684 individuals from Eurasia and the USA and found 41 different haplotypes. Our comparison between the native and invasive ranges found little reduction in measures of allelic richness (A) and haplotype diversity (h) since introduction into the USA, as well as little evidence of regional geographic structuring of haplotypes in neither Eurasia nor the USA. Assignment tests indicated most likely origins of many invasive haplotypes, though some origins were found outside of the putative native range of the species. Exact locations are identified for a diverse set of invasive haplotypes which can be used in ongoing host-specificity tests of potential biological control agents. Biological control efforts should be prepared to contend with a genetically diverse invasion derived most likely from multiple Eurasian origins.